1. Field of the Invention
The present invention relates to a semiconductor memory, and more particularly to a memory device having a non-volatile function known as a non-volatile random access memory.
2. Description of the Prior Art
A non-volatile random access memory is a memory which has a conventional random access memorizing function and a non-volatile memorizing function. The non-volatile random access memory operates as a random access memory under normal operating conditions. However, when a driving voltage supplied to the memory device drops or when the power supply is turned off, the information stored in the random access memory circuit is transferred to a non-volatile memory circuit. Thereafter, when the driving voltage recovers or when the power supply is turned on, the information is returned to the random access memory circuit. Thus, the information is kept in the memory without destruction for a long period of time.
In the prior art, many transistor elements are required to constitute the non-volatile random access memory cell. Therefore, it is hard to provide a large capacity memory device. On the other hand, a non-volatile random access memory cell with a small number of transistor elements is proposed in U.S. Pat. No. 4,207,615. This memory cell has a non-volatile MOS transistor between one of a pair of load transistors and one of a pair of driver transistors in a flip-flop type RAM memory cell, and further a depletion transistor is added in parallel to the non-volatile MOS transistor. Therefore, impedance mismatching occurs between the two load stages in the flip-flop cell. As a result, the reading and writing speeds in an ordinary random access operation become slow. Furthermore, the proposed memory cell has a shortcoming in that it is difficult to form the cell by means of a complementary MOS (C-MOS) circuit. Therefore, the power consumption of the overall circuit is unavoidably large. Further, in U.S. Pat. No. 4,132,904 two non-volatile MOS transistors are used as driving MOS transistors in a flip-flop memory cell. However, according to this memory cell, reading operations and writing operations are very complex. Namely, the prior art non-volatile memory devices have disadvantages in that many elements are required to constitute the memory cell and in that complicated operations are required.